The present invention relates in general to color cathode ray tubes. More specifically, the present invention is directed to a color cathode ray tube having a mask frame, a shadow mask attached to the mask frame and an inner shield attached to the mask frame, in which at least one of these parts includes iron as its chief element and has an oxidized layer on its surface.
Generally a color cathode ray tube has a mask-frame, a shadow mask and an inner shield within an envelope. These parts include iron as a chief element. Oxidized layers are formed on their surfaces. These oxidized layers are formed so as to prevent the parts from being rusted until they are assembled in the tube, and to increase the efficiency of heat radiation and also reduce the reflection of an electron beam during the time when the cathode tube operates. The oxidized layers are formed by heat treatment in an atmosphere containing oxygen after press forming. The oxidized layer is generally a bluish black color. As a result, the oxidized layer is sometimes referred to as a blackened layer.
However, the oxidized layer causes a problem. It is weak in adhesion and it easily peels off. Peeled off oxidized particles affect the tube function seriously, such as by filling up the aperture of the shadow mask or causing a discharge to occur between tube electrodes.
Therefore, several techniques for improving adhesion of the oxidized layer have been proposed. For example, Japanese patent disclosure No. 8,963/1979 discloses reducing a silicon content of silicon added as deoxidizing agent during manufacturing of the raw steel material used to manufacture the iron parts of the tube and which remains inevitably in the steel to less than 0.03%. As another example, Japanese patent disclosure No. 90,033/1980 discloses washing the shadow mask with water containing chloric ions in an amount less than 20 ppm before oxidizing. These methods improve adhesion of the oxidized layer, however, they do not provide adequate improvement to completely overcome the problems associated with the oxidized layer.